1. Field of the Invention
The present invention relates to an optical modulator and a design method therefor. More particularly, the present invention relates to an optical modulator that utilizes an electro-optic effect of a crystal substrate. The invention also relates to a method of designing an optical modulator which performs electrical-to-optical conversion by modulating a given light beam with an electrical signal.
2. Description of the Related Art
Recent years have seen an increasing use of multimedia applications, with a growing awareness of demands for more advanced optical communications networks that provide higher speeds and larger bandwidths. Optical modulators are one of the key devices for realizing such high-performance optical networks. One type of optical modulator is an external modulator, which performs electrical-to-optical conversion by modulating an incoming light beam with an electrical signal. The modulating signal produces an electric field across an optical waveguide fabricated on a substrate, so that the light beam propagating along the waveguide will be varied in phase as a result of interaction between the light and the electric field being applied to it.
To meet the recent demand for high-speed, high-bandwidth optical communication, technological migration from 10 Gbps-class systems to 40 Gbps-class systems has begun, Including the deployment of dense wavelength-division multiplexed (DWDM) optical transmission systems. The new systems require optical modulators to operate four times faster than before. To fulfill this requirement, it is necessary to reduce the drive voltage of modulators since high-speed electronic circuits cannot produce a large voltage swing.
In designing such an external optical modulator as mentioned above, however, we trade off fast r operating rates (or wider modulation bandwidths) against lower drive voltages. In general, w can increase th modulation rates if the electric capacitance is small. This would be accomplished by simply cutting the length of the optical waveguide (or actually, reducing the length of a signal electrode that makes a modulating electric field interact with the light beam traveling on the optical waveguide). The reduction of this “interaction length,” however, also reduces the amount of resulting phase displacements, causing a decreased modulation ratio. Contrary to our desire for a lower drive voltage, we now have to increase the drive voltage to yield a sufficient modulation depth. For this reason, there have been difficulties in further improving the performance of conventional optical modulators or reducing the drive voltage for optical modulators.